In the recent years, the necessity of rendering the steelmaking processes more efficient, more productive, and less wasteful, has become increasingly urgent, due to rising production costs (particularly energy costs) and also due to the increasing restrictions imposed upon steel plants for ecological reasons.
One of the successful routes for producing steel, which is being increasingly promoted and has become more widely utilized, is the direct reduction without melting of lumps or pellets of ore with subsequent electric arc furnace refining. This is in contrast to the traditional steel plants using blast furnaces and basic oxygen converter furnaces for making steel from melted ore. Although in the present specification a preferred embodiment of the invention is described as applied to a steelmaking plant comprising a direct reduction plant and electric arc furnaces, it is evident that the invention in its broader aspects can be adapted to other applications where DRI or other metallic iron bearing particles (hot or cold) are to be transported.
In order to better understand the invention, some of the characteristics of DRI are given herein, which will help one to appreciate why pneumatic transport systems had never been previously recommended for commercialized DRI nor ever successfully used before with this material.
DRI is commercially produced by direct reduction of lumps or pellets of iron ore by contacting these with a stream of reducing gas at a high temperature. Reduction is carried out in the solid state. The resulting DRI is a friable particulate solid which is very porous and reactive at high temperatures. At such elevated temperatures, it reoxidizes exothermically with water or oxygen in the ambient air.
Iron ores employed for direct reduction are generally iron oxides: hematite and magnetite. When the iron ore has a high iron content, for example above 55%, it can be economically processed by simply breaking it down to particle sizes at least 80% and preferably at least 90% of which are greater than 0.5 cm and range up to about 6 cm (i.e. 0.2 to 2.4 inches). It is also necessary that said ore has a high mechanical strength so as to withstand pressures, shocks and crushing which tend to create undesirable and excessive fines during its transport and handling. Such dust-like fines can cause considerable problems, such as uneven flow and gas channelling inside the fixed or moving bed reduction reactor. This results in unhomogeneous reduction, thus producing DRI of an inferior and uneven quality.
In order to render it mechanically stronger, it is preferred in many cases, to grind the iron ore, magnetically concentrate it to increase its iron content, include some additives such as lime and dolomite, and form it into pellets in the 0.5 to 2.4 cm range. In this pelletizing process, the pellets formed are generally spherical with the desired chemical composition needed for optimal performance in the reduction process and also in the subsequent steelmaking stage in the electric arc furnace. Since pellets have a higher iron content with a more uniform quality, they can consequently be transported over long distances by truck, rail, etc., more economically (because the unnecessary transportation of a substantial amount of gangue is avoided).
Direct reduction plants chemically reduce iron ores by contacting the particles, which may be irregularly shaped pieces of iron ore or pellets or mixtures thereof, with a stream of reducing gas, largely composed of hydrogen and carbon monoxide, at a temperature between 850.degree. and 1,050.degree. C., normally at about 950.degree. C.
The commercial reduction reactor may be of the fixed bed or moving bed type. It is evident that in order to increase the reaction rate between the solid ore particles and the reducing gas, it is desirable that said particles be highly porous. However, this characteristic also makes DRI very reactive, since it contains a high proportion of metallic iron, which tends to reoxidize when in contact with the oxygen in air or water. As is typical of all chemical reactions, reoxidation of DRI is faster and more violent as the temperature increases. This is why DRI in the past always has most desirably been safely handled at ambient temperature and is normally cooled down inside the reduction reactor by circulating a cooling gas therethrough before it is discharged. See, for example, U.S. Pat. Nos. 3,765,872; 4,046,557; and 4,150,972.
Cooling DRI, although commonly necessary for the safe handling of the DRI to avoid reoxidation problems and to reduce the need for expensive temperature resistent pumps, valves, etc., is however disadvantageous with respect to the energy efficiency of the overall steelmaking process. Since much of the thermal energy of the DRI is lost through its cooling; therefore, it is necessary to spend more energy to heat the DRI once again in order to melt it and convert it to steel. Only some of the lost energy is usefully recovered in steam generation, in heat exchangers, and the like.
To meet this problem, it has been proposed in the past to omit cooling the DRI in the reduction reactor and to discharge it at the highest feasible temperature, normally in the range of 400.degree. to 750.degree. C., in order to decrease energy cost in the electric furnaces where DRI is melted, or alternatively to hot briquet the DRI, commonly at temperatures above 600.degree. C. This represents substantial energy savings. However, transportation of DRI at high temperature has so far involved using systems which have significant drawbacks.
U.S. Pat. Nos. 3,799,367 and 4,188,022 teach discharging DRI at a high temperature from a direct reduction reactor without cooling it down to ambient temperature. It is proposed to transport this hot DRI to its next processing step, utilizing containers which are placed at the outlets of the discharge bins of the reduction reactor. These containers are filled with an inert gas to prevent contact of hot DRI with oxygen in the air, thus avoiding reoxidation thereof. This transport system, however, presents a number of disadvantages, because the containers with DRI must be moved through the plant by means of trucks or railroad equipment. This requires a maintenance system for motorized vehicles with its consequent high operating costs.
Furthermore, such a system also needs transit space within the plants. In an already existing plant, it is very difficult to open suitable space for transit of trucks or a railroad, if such installations are not planned beforehand.
As previously discussed, it has been also proposed in the past to form the DRI particles into briquettes while at high temperature whereby compaction of DRI particles into dense briquettes decreases substantially the porosity and thereby its reactivity. However, even if only for hot DRI transport to the briquetting press, systems currently in use are complicated and involve large investment and high operating costs.
German patent No. 3806861 teaches transport of hot DRI in a bin. Such bins are used with pressure locks in some direct reduction processes operating at pressures over one atmosphere. These processes require pressure locks for charge and discharge of the reduction reactor. This transport system is however applicable only to short distances, for example from the reduction reactor to a briquetting press located close to said reactor. If DRI is to be transported over longer distances, for example hundreds of meters, this system using pressure lock bins is not practical nor economical since it would require a larger number of such bins, which, due to their required characteristics for pressure, temperature and abrasion resistance as well as their sealing valves, are expensive.
By far the most common current practice for DRI transportation involves discharge from the reactor at low temperatures, for example at temperatures below 100.degree. C., and utilization of open belt conveyors. DRI is moved by means of these belt conveyors, to silos or storage bins and then to feeding bins of the electric furnaces at the meltshop. This method, however, has several drawbacks. For example, the fines, e.g. the very small particles produced from the abrasion and breakage of ore lumps, of pellets, or of DRI inside the reduction reactor and during DRI handling, are spilled at transfer points and are entrained by ambient air currents passing over the open conveyors with the consequent problems of losses of valuable metallic iron units and of significant environmental pollution. These losses of metallic iron, which mainly occur at the transfer stations of the DRI, can be as 2% to 10% of DRI production, depending on the type of facilities.
Non-commercialized attempts to produce DRI by fluidized bed direct reduction methods have been proposed from time-to-time. These teach the use of very fine-grained ores up to only 3 mm in diameter and preferably less than 0.5 mm). In dealing with a grain size appropriate to being fluidized, a few such references have suggested pneumatic transport of such "fine-grained sponge iron" (see U.S. Pat. Nos. 4,007,034 and 4,045,214). However, U.S. Pat. No. 4,412,858 is the only reference known to applicants which is relevant to the commercially-proven larger-sized DRI particles (i.e. greater than 0.5 cm) that has any suggestion of pneumatic transport of DRI. Yet, even this latter patent's teaching is only in the context of the larger "sponge iron pellets [being]. . . converted to finely divided form" by "grinding or milling" prior to transport by a carrier gas.
Crushed limestone is reported to have been pneumatically transported over short distances as a feed device; however, this is a relatively soft material as compare to DRI (or even to iron ore). Thus, in spite of such uses, such lime transport has never been extended to or suggested for iron ore or for DRI of a size greater than 3 mm.
There is also considerable literature on pneumatic transport of catalyst particles in the petroleum industry, but always of smaller particles of a size appropriate for use in a fluidized bed.